1. Field of the Invention
The present invention relates to an information communication network system built on a tree shaped information communication network represented by a bidirectional CATV or a star shaped information communication network represented by an optical fiber system network (also called "FTTH" (fiber to the home)) in order to simultaneously accomplish a connection-less high speed information communication network service and/or also a connection oriented type telephone and data service similar to LAN (Local Area Network), a central information communication control device and information communication terminals both used in this information communication network system, in addition, an information transmission method and modulation method.
2. Description of the Related Art
The recent popularity of personal computers (hereinafter simply called "computers" or "PC") is very remarkable. The use of those computers is rapidly expanding to high-grade information communication services such as a so-called PC on-line service and Internet, not mentioning their traditional usage as individual computer units. The currently available means for connecting general subscribers to such a PC on-line service and Internet is only a connection oriented type analog telephone line or a narrow-band ISDN digital line. The analog telephone line can merely provide a service at a transmission rate of 2.4 kbps to 28.8 kbps via modems, while the narrow-band ISDN can merely provide a service at a transmission rate of 16 kbps or 64 kbps. This limitation is becoming a big bottleneck in receiving or enjoying high-grade information communication services such as accessing via a network to information like documents using multimedia technology that is recently advancing remarkably.
In a limited range of usage within buildings or in a LAN, high speed information communication at a rate of 10 Mbps or faster using a connection-less LAN is available so that such information communication has become popular from large-sized companies to small or medium-sized firms. There is however no specific means which allows a connection-less high speed information communication service corresponding to LAN to be used as one of public information communication services in home, and the development and practical usage of such specific means are strongly demanded.
To realize a connection-less high speed information communication service as one public information communication service at a low cost, it is essential to more properly and efficiently provide communication resources needed for the service. In view of the recent noticeable development of server-client systems, it is typical that a client computer retrieves information stored in a server. In this case, it should be noted that the communication is asymmetrical transmission wherein the amount of information transferred downward to a client computer from the server is far greater than the amount of information transferred upward to the server from the client computer. In the Ethernet which is a typical type of LAN, for example, information transmitted from one terminal is sent to all the terminals. If this transmission system is directly used as a public information communication service, information on an upward transmission line which has been transmitted from a client is transmitted directly on downward transmission lines. This system wastes the communication resources of the downward transmission line accordingly. This means that the technology cultivated in the LAN field cannot be adapted directly to public information communication services.
Access system networks, which can utilize high speed information communication services at a transmission rate of 10 Mbps or faster as public information communication services, include a so-called CATV network which is very popular in the U.S.A. and is expected to become popular in Japan too, an HFC (hybrid fiber and coaxial) which is realized by replacing a part of this CATV network with an optical fiber system, and an optical fiber system network (FTTH: fiber to the home) which is expected to be put to a practical use soon. The technologies that can be adapted to those networks are important in quickly accomplishing the mentioned services.
What is common to those networks is the use of tree shaped or star shaped bidirectional transmission lines separated to upward transmission lines and downward transmission lines (lines may be separated physically or the same line may be separated by frequencies or wavelengths) as shown in FIGS. 1 and 2. It is also important that the technologies should consider the performances of those transmission types.
If a connection oriented type telephone and data service can be provided at the same time as the aforementioned connection-less service is accomplished, a business firm who intends to present those services can increase the number of computer communication users and the number of telephone users without double investments. It can therefore be expected that users can receive both transmission services at lower costs and that a new service rendered by computers linked to telephones, such as users making a telephone communication while seeing the same information through computers.
To provide a connection-less high speed information communication service in a tree shaped or star shaped network, various matters should be dealt with, such as fairness, no lost packet, high speed and high throughput, upward ingress noise, poor quality of downward transmission, long network length and information leakage.
The "fairness" is to give a fair transmission permission to computers as information communication terminals wherever they are located in a network. In a contention system as CSMA/CD (carrier sense multiple access with collision detection) that is used in the Ethernet, for example, the following case may occur when the levels of collided signals differ significantly from each other in a predetermined access control method. When high-level signals collide, the collision is not detected and a packet will properly reach the destination undestroyed by the collision. When low-level signals collide, on the other hand, the collision is detected or a packet is destroyed and a computer tries a resend so that it becomes difficult to obtain the transmission permission. The difference between those signal levels is caused by the position of connection of a computer to the transmission line or the chronological change in the output device even if the levels of signal output from that computer are equal. A computer connected to the position of a large attenuation or a computer whose output signals have attenuated levels due to the chronological change in the output device is disadvantageous over other computers.
The "lost packet" may occur when a received packet contains many bit errors so that the sender address cannot be discriminated and the receiver cannot request the sender to resend the packet. More specifically, the sender tries a resend if there is no response from the receiver when a given time (e.g., 30 seconds) has passed. This phenomenon considerably deteriorates the throughput.
The "high speed and high throughput" indicates, for example, the Ethernet (maximum length of 2.5 km) which has a transmission rate of 10 Mbps and has higher throughput characteristics than the conventional CSMA system. This performance should be exhibited also on a transmission line having a significantly long network length (maximum length of about 15 km or more) like that of CATV.
The "upward ingress noise" is an electromagnetic wave (noise) like private band which occurs upward of a connector provided at the end of a coaxial cable when the connector is open or loose. As such noises occur at individual sections to be mixed together, the noise level increases, thus eventually deteriorating the quality of the upward transmission.
The "poor quality of downward transmission" means that the noise environment associated with the downward transmission becomes poor temporarily or steadily. This phenomenon occurs when cables, etc., on an old established site have been damaged due to the chronological usage so that noise is likely to enter from outside or the noise index of the trunk amplifier for compensating for signal attenuation becomes poor, or if a vast interference source like a radar is located nearby even when cables are new.
The last item, "information leakage," means that there is a possibility of tapping and unauthorized usage of information because the signals to be transmitted (broadcasted) downward are receivable by any computer.
To ensure the simultaneous information transmission and telephone service should face a great technological challenge of accomplishing both connection-less and connection oriented type communication systems, which are quite different in performance, on the same system without impairing their own characteristics.